When It Rains, It Pours

Global warming is happening now and its effects are being felt in the United States and around the world. Among the expected consequences of global warming is an increase in the heaviest rain and snow storms, fueled by increased evaporation and the ability of a warmer atmosphere to hold more moisture.

An analysis of more than 80 million daily precipitation records from across the contiguous United States reveals that intense rainstorms and snowstorms have already become more frequent and more severe. Extreme downpours are now happening 30 percent more often nationwide than in 1948. In other words, large rain or snowstorms that happened once every 12 months, on average, in the middle of the 20th century now happen every nine months. Moreover, the largest annual storms now produce 10 percent more precipitation, on average.

An increase in extreme downpours has costly ramifications for the United States, with the potential to cause more flooding that jeopardizes property and lives. With scientists predicting even greater increases in extreme precipitation in the years ahead, the United States and the world must take action to reduce pollution that contributes to global warming.

Extreme rainstorms and snowstorms are happening more frequently.

Extreme downpours  rainstorms and snowfalls that are among the largest experienced at a particular location*  are now happening 30 percent more often on average across the contiguous United States than in 1948. (Our analysis covered the period from 1948 to 2011, which offered the most complete weather data.)

New England has experienced the greatest change, with intense rainstorms and snowstorms now happening 85 percent more often than in 1948. The frequency of intense rain or snowstorms nearly doubled in Vermont and Rhode Island, and more than doubled in New Hampshire. (See Figure ES-1.)

The change has also been pronounced in the Mid-Atlantic, the South, the Midwest and the Mountain West. New York, Pennsylvania and Missouri each experienced an increase in extreme downpour frequency of more than 50 percent.

In total, 43 states showed significant** increases in the frequency of extreme downpours. Only one state, Oregon, experienced a significant decrease. (See Table A-2 on page 34 for full data by state.)

Global warming  driven by pollution from the combustion of fossil fuels  is helping to fuel the increasing severity of downpours.

The U.S. Global Change Research Program  composed of a wide range of leading experts from the U.S. National Oceanic and Atmospheric Administration, the National Aeronautics and Space Administration, and universities  called the increase in heavy downpours one of the clearest precipitation trends in the United States and linked the phenomenon to global warming in its report, Global Climate Change Impacts in the United States.

The average temperature in the United States has increased by 2° F over the last 50 years. Nine of the ten warmest years on record have occurred since 2000.

Warmer temperatures increase evaporation and enable the air to hold more water. Scientists have found that the water content of the atmosphere is now increasing at a rate of about 1.3 percent per decade. The additional moisture loaded into the atmosphere by global warming provides more fuel for intense rainstorms and snowstorms.

Global warming will very likely drive future increases in extreme downpours, with a wide range of harmful consequences.

Experts at the U.S. Global Change Research Program project that heavy downpours are very likely to become more frequent and more intense with further warming. Heavy downpours that are now 1-in-20 year occurrences are projected to occur about every 4 to 15 years by the end of this century, according to their report, while producing 10 to 25 percent more precipitation per storm, depending on location and on the scale of future emissions of global warming pollution.

Extreme rain and snowstorms can harm people and property  primarily by increasing the risk of flooding. In 2011, floods killed more than 100 people and caused more than $8 billion in damage to property and crops.

Bigger and heavier rainstorms and snowstorms will not necessarily lead to more water being available for ecosystems or human use. Indeed, scientists warn that some areas of the country may experience both heavier extreme rainstorms and more frequent and severe drought, due to higher evaporation of soil moisture and longer dry spells between significant rainstorms.

To protect our communities, our safety, and our environment, we must rapidly and substantially reduce pollution that causes global warming.

Federal and state governments should adopt and implement limits on global warming pollution capable of reducing emissions to at least 35 percent below 2005 levels by 2020 and by at least 85 percent by 2050. These emission reductions are broadly consistent with what science tells us is necessary to lessen the most costly and devastating consequences of global warming.

Short of economy-wide limits on global warming pollution, local, state and federal governments should focus on reducing pollution from the largest sources  most notably power plants and transportation. The Obama administrations proposed carbon pollution standards for new power plants and the proposed carbon pollution and fuel efficiency standards for cars and light trucks through model year 2025 are two noteworthy proposals at the federal level. Regional programs such as the Northeasts Regional Greenhouse Gas Initiative can also help to achieve this goal.

The United States - including federal, state and local governments - should adopt clean energy solutions that reduce our dependence on fossil fuels and emissions of global warming pollution. Among the most important steps are:

Adopting enforceable targets, financial incentives, regulatory changes and investment strategies that increase the use of renewable energy sources such as wind and solar power.

Continuing to develop and implement the fuels and technologies of the future  from electric vehicles to energy storage devices to smart grid technologies and new renewable sources of energy  through government support of research, development and deployment of those technologies and the adoption of technology-forcing standards where appropriate.

Federal, state and local officials should take steps to better protect the public from the consequences of extreme weather events  steps that save costs compared to suffering the full brunt of these extreme events. Government officials should explicitly factor the potential for global warming-induced changes in extreme weather patterns into the design of public infrastructure and revise policies that encourage construction in areas likely to be at risk of flooding in a warming climate.

*Specifically, we defined extreme events as those expected to occur no more than once per year on average at a particular location based on the historical record. In other words, we identified the 64 events at each weather station with the largest 24-hour precipitation totals across the 64-year time period of the study, and labeled them extreme.

**The term significant here indicates a very high probability that the trend is real and not simply the result of chance, based on statistical analysis. Specifically, significant results showed 95 percent or greater probability that the slope of a best-fit line through the data was greater than zero, supporting the conclusion that the frequency of extreme downpours has increased.